The present invention relates to a novel oxide garnet single. crystal or, more particularly, to a rare earth-based oxide garnet single crystal having improved light absorption characteristics in the 1.55 .mu.m wavelength band and outstandingly small saturation magnetization and useful as a functional element in various magneto-optical devices such as optical isolators, optical switches and the like, which enables a more compact design of the devices.
Single crystals of a bismuth-substituted rare earth-iron oxide garnet are well known materials to serve as a functional element in various kinds of magneto-optical devices such as optical isolators, optical switches and the like and they are conventionally prepared by the method of epitaxial growth on the substrate of a certain single crystalline material. As a consequence of the epitaxial growing method, it is almost unavoidable that the rare earth-based oxide garnet single crystals as grown are contaminated with various kinds of contaminants including, for example, lead ions originating from the lead oxide used as a constituent of the flux of the melt and platinum ions originating from the material of the crucible used in the liquid epitaxial growing method. These contaminants are of course very detrimental against the performance of the single crystals as a functional element of magneto-optical devices. When the oxide garnet single crystal is used as an element in an optical isolator, for example, the absorption of light in the working wavelengths of 0.8 .mu.m, 1.3 .mu.m and 1.55 .mu.m is more or less increased by these contaminants resulting in an increased insertion loss.
In consideration of these problems, a proposal has been made to introduce a very trace amount of divalent or tetravalent metallic ions such as Ca.sup.2+, Mg.sup.2+, Ti.sup.4+ and the like into the oxide garnet single crystals of this type (see, for example, The 11th Japan Applied Magnetics Society, Preprints for Scientific Lectures, November 1987, 2C-10, page 137). This method has another problem that, while the rare earth-based oxide garnet single crystals as epitaxially grown are required to have a thickness of at least 50 .mu.m, introduction of the above mentioned metallic ions thereinto causes variation in the chemical composition of the oxide garnet layer during the epitaxial growing to destroy the uniformity in the performance of the single crystal-based devices.
Alternatively, Japanese Patent Publication No. 5-13916 teaches that the above mentioned problem can be solved with an epitaxially grown oxide garnet single crystal having a chemical composition expressed by the formula EQU (Bi.sub.0.26 Eu.sub.0.07 Tb.sub.0.67).sub.3 (Fe.sub.0.94 Ga0.06).sub.5 O.sub.12.
Although this oxide garnet single crystal has good magneto-optical characteristics in the 1.3 .mu.m wavelength band, the light absorption loss in the 1.55 .mu.m wavelength band is increased due to the influence of the light absorption by the terbium ions in the longer wavelength region so that, for example, magneto-optical devices such as optical isolators prepared by using such a single crystal suffer from an increased insertion loss when they are to be used in the 1.55 .mu.m wavelength band.
Further alternatively, Japanese Patent Kokai No. 2-77719 proposes a magneto-optical oxide garnet single crystal suitable for temperature compensation, which has a chemical composition expressed by the general formula Ho.sub.3-x-y Gd.sub.x Bi.sub.y Fe.sub.5-z Ga.sub.z O.sub.12, in which x is a positive number of 0.8 to 1.2, y is a positive number of 0.7 to 1.2 and z is a positive number of 1.2 to 1.7. As a consequence of the substitution of the so large amount of gallium as a non-magnetic element for iron as the magnetic element, the Faraday rotation coefficient thereof is so small as to be about 590 degrees/cm for the light of a wavelength of 1.55 .mu.m as is shown in the Examples so that such an epitaxially grown garnet film cannot be used alone in a Faraday rotation device such as optical isolators unless the film thickness of the garnet film is unduly large. Needless to say, such a garnet film having a sufficiently large thickness can be obtained only by a substantial extension of the time taken for the epitaxial growth of the garnet film naturally with a disadvantage in the production costs.
Still alternatively, Japanese Patent Kokai No. 3-280012 proposes a magneto-optical material which is a magnetic oxide garnet single crystal film having a chemical composition expressed by the formula Ho.sub.3-u-v Gd.sub.u Bi.sub.v Fe.sub.5 O.sub.12, in which u and v are each zero or a positive number smaller than 3 with the proviso that u+v does not exceed 3. A problem in the garnet single crystals of this type is that, when the content of gadolinium is so small as in the garnet of the formula Ho.sub.1.45 Gd.sub.0.25 Bi.sub.1.30 Fe.sub.5 O.sub.12 shown in the Examples, the magnetic saturation thereof is as large as about 1000 G so that a magneto-optical device such as optical isolators prepared with such a garnet single crystal can work only by using an unduly large magnet generating a magnetic field strong enough.